Exposure assessment, an essential component of environmental risk analysis, requires quantitative approaches for estimating the distribution and persistence of pollutants in environmental media. Volatile compounds (VOCs), such as TCE, DCE, toluene, p-nitrophenol, and naphthalene are on the EPA Priority Pollutants List and are commonly found at Superfund waste sites. Metam sodium is a fumigant used in large quantities in agriculture. From initial placement in the vadose zone, these volatile contaminants move into the groundwater and the air which are the two most important exposure routes for humans. While in the soil, they may be transformed by microbial metabolism into compounds which are either more or less hazardous than the original contaminants. Studies will be conducted to investigate the fundamental processes including diffusive and convective transport, sorption/desorption, and biodegradation that affect the fate and transport of both the liquid and vapor phases of VOCs in soil and geological sediments. Research will focus on the coupled and interdependent physical, chemical, and biological processes determining the transport and distribution of chemical mixtures. Laboratory-scale, column studies will be conducted using soil and geological sediments to determine the effect of multiple environmental processes on the potential for movement of VOCs to the groundwater and atmosphere. Persistent metabolites resulting from biodegradation will be identified with the assistance of the Immunochemical project and the Analytical Core. Estimates of vapor concentrations. likely for various waste site scenarios will be provided to investigators in the thermal remediation project. Samples of dust will be provided to the pulmonary exposure project in order to assess the potential health hazard of chemicals sorbed to dust. Chemicals and metabolites in leachate from soil will be tested for toxicity using the medaka and cell based bioassays. Relationships between VOC transport processes and field-scale geological attributes of the vadose zone such as depositional facies, grain size, organic carbon, sorting and mineralogy will be determined. Quantitative descriptions of these relationships will be incorporated into deterministic and stochastic models of transport in the vadose zone which include two and three-dimensional numerical modeling experiments of flow and transport of a single liquid phase (VOC dissolved in water) and a gaseous phase. A heat-water transport model for the vadose zone, which can be used under conditions of strong diurnal land surface heating, will be developed and tested with laboratory and field experiments. This model will be coupled with the models of chemical volatilization to obtain the flux into the atmosphere. The validation of the atmospheric component will be based on lidar measurements of the boundary layer over 100 m scale plots up to regional scales of a few kilometers. These models will be used to evaluate potential human exposure levels at the field scale.